WEDNESDAY, June 12 (HealthDay News) -- An antiviral drug may help protect injection drug users from HIV infection, a new study finds.

The study of more than 2,400 injection drug users recruited at 17 drug treatment clinics in Thailand found that daily tablets of tenofovir reduced the risk of HIV infection by nearly 49 percent, compared to inactive placebo pills.

One expert said an intervention to help shield injection drug users from HIV -- the virus that causes AIDS -- is much needed.

"This is an important study that opens up an additional option for preventing HIV in a hard-to-reach population," said Dr. Joseph McGowan, medical director at the Center for AIDS Research & Treatment at North Shore University Hospital in Manhasset, N.Y.

He noted that "HIV infections continue to occur at high rates, with over 2.5 million worldwide and 50,000 new infections in the U.S. each year. This is despite widespread knowledge about HIV infection and the way it is spread, through unprotected sex and sharing needles for injecting drugs."

The participants included in the new study were followed for an average of four years. During that time, 17 of the more than 1,200 patients taking tenofovir became infected with HIV, compared with 33 of an equal number of patients taking a placebo, according to the study published online June 12 in The Lancet.

Further analyses of the results showed that the protective effect of tenofovir was highest among those who most closely followed the drug's prescribed regimen. In this group, the risk of HIV infection was reduced by more than 70 percent, said study leaders Dr. Kachit Choopanya and Dr. Michael Martin, chief of clinical research for the Thailand Ministry of Public Health--U.S. Centers for Disease Control and Prevention Collaboration.

Prior research has shown that preventive use of antiviral drugs cuts the risk of sexual transmission of HIV in both heterosexual couples and men who have sex with men, and also reduces mother-to-child transmission of HIV. But this is the first study to show that this approach might also be effective among injection drug users.

Worldwide, injection drug use is believed to cause one in 10 new HIV infections. But rates of infection associated with injection drug use are far higher in some areas of the world, such as eastern Europe and central Asia. In these regions, up to 80 percent of new HIV infections are caused by injection drug use.

According to McGowan, tenofovir is no "silver bullet" that would, on its own, eliminate the risk of HIV infection for drug abusers. But it could be a key ingredient in reducing the odds.

"Adoption of this strategy, not as a stand-alone, but in conjunction with needle exchange, counseling, opiate substitution, social support and mental health therapy may enable us to get ahead of this expanding epidemic," McGowan said.

He added that the participants in the study were also provided with what's known as "directly observed therapy," where the drug is administered under the observance of a health care worker. Services like this, along with monthly HIV testing and condom distribution, might not occur in "real life" treatment situations, McGowan said, so outcomes might not be as good as in this clinical trial.

Another expert agreed that adherence to tenofovir therapy is key to success.

Tenofovir "accumulates slowly [in the body], making the case for adherence -- which is strongly associated with the efficacy of the drug," said Victoria Richards, assistant professor of medical sciences at the Frank H. Netter MD School of Medicine at Quinnipiac University, in Hamden, Conn.